Liquid crystal display

Abstract: A liquid crystal display which comprises an active matrix circuit board equipped with a plurality of transistors having a plurality of pixel electrodes (511,512,513,521,522,523) arranged along a plurality of lines and rows (411,412,413,421,422,423) and drains connected to these pixel electrodes, scanning lines (21,22) for commonly connecting gates (111,112,113,121,122,123) of the transistors arranged along the lines, and signal lines (11,12,13) for commonly connecting sources (311,312,313,321,322,323) of the transistors arranged along the rows, a substrate having a facing electrode (8), and a liquid crystal interposed between the active matrix circuit board and the substrate the liquid crystal display being constituted so that a voltage may be applied to regions (211,212,213,221,222,223) of a conductive mode which is different from a conductive mode of the sources and the drains of semiconductor layers constituting the transistors.

Description: The present invention relates to a liquid crystal display panel having an active matrix substrate, and an active matrix substrate therefor. Heretofore, there has been suggested a liquid crystal display for displaying, for example, television images by the use of a high-density two-dimensional matrix liquid crystal display (an active matrix type liquid crystal display) in which a thin film transistor is utilised. Under such circumstances, as a technique for realising the liquid display of a high contrast by using the active matrix type liquid crystal display, a system has been prevalent in which pixel signals are written by the use of thin film transistors (poly-Si TFTs) in which semiconductor layers comprising a polycrystalline silicon (poly-Si) are utilised. However, the liquid crystal display in which switching elements comprising the poly-Si TFTs are used has the following problems. (1) Since the poly-Si has a low carrier mobility, a high-speed switching operation is difficult, so that the number of the pixels for the liquid crystal display is limited, with the result that the liquid crystal display having a high resolution cannot be obtained. (2) A leakage current flows through grain boundaries of the poly-Si between a source and a drain, and so a predetermined voltage cannot be held between pixel electrodes. In consequence; the contrast and gradation deteriorate. (3) The size, the orientation and the like of the poly-Si grains cannot be uniformly controlled, so that they are different in the respective TFTs. Thus, the threshold values of the transistors are uneven. In addition, change in properties with time also occurs. For these reasons, it is difficult to design a circuit or a panel using the TFTs. In order to solve the above-mentioned problems, improvement has been made, for example, by enlarging the grain size of the poly-Si to heighten the carrier mobility from a conventional level, i.e., several-tens level (n-type TFT) to tens-hundreds level.However, if the grain size is enlarged and crystallinity is increased, the following problems arise. (4) The life of an impact-ionized carrier (hot electron or hole) generated by a high electric field at the drain terminal of the TFT is prolonged, and such hot carriers lead to a change of the potential in a TFT channel, whereby a leakage current of the TFT is generated. (5) Furthermore, in order to relieve the above-mentioned high electric field, a silicon (Si) layer constituting the TFT is required to be thickened, and in such a case, the leakage current caused by a defect of an Si side wall of the TFT increases. Moreover, when the TFTs are used in the display, a leakage photo-current caused by light leaked into the pixel TFTs is more easily generated. Japanese Patent Application JP-A-63316027 and Patent Abstracts of Japan Vol. 13, No. 157 (p-857) discloses an active matrix substrate in which each pixel thin film transistor MOSFET has a signal gate electrode adjacent the substrate and another insulated gate electrode on the opposite side of the transistor channel. This other gate electrode is of opaque material and acts as a light shield. It is connected to a source of fixed voltage to stabilise the TFT operation. Japanese Patent Application JP-A-02178965 and Patent Abstracts of Japan vol. 14, No. 448 (E-0983) discloses a general purpose semiconductor-on-insulator insulated gate field effect transistor in which the active layer is partitioned into highly doped source and drain regions each side of a lightly doped channel of the opposite conductivity type. The channel has an embedded conductor which in use is connected to a source of fixed voltage to stabilise the transistor operation. The active matrix substrate of the present invention, for use in a flat panel liquid crystal display, is of the type comprising: a substrate having thereon a plurality of pixel electrodes, each connected to the drain of a respective field-effect transistor defined therein, and arranged by row and by column in a matrix array; respective scanning lines, arranged along each column, connected to the gates of the transistors of the respective columns; and respective signal lines, arranged along each row, connected to the sources of the transistors of the respective rows. In accordance with the present invention, this substrate is characterised by: a layer of semiconductor having a high density of dopant impurities, which is arranged around the periphery of, and in contact with, semiconductor layer portions forming the field effect transistors; which layer of semiconductor is of conductivity type opposite to that of the source and drain of each transistor, and is provided with means of connection to a source of fixed voltage. In the liquid crystal display of the present invention, hot carriers generated in a high electric field or light photo-carriers generated owing to light leakage flows to a power source connected to the wells, so that the fluctuation of a well potential does not occur. In consequence, a TFT leakage current is much smaller as compared with a conventional case, so that the signal of the pixel electrodes does not change, thereby realising a high contrast and a high gradation. Preferably plates, each being one of two plates constituting a capacitor, are connected to the drains of the transistors, so that the potential change of the pixel electrodes can be further restricted by the capacitors, even if a similar leakage current of the TFTs is generated. In consequence, high contrast and high gradation can be realised. In general, if such a constitution is employed, connections to the power source increase to deteriorate an aperture ratio. In the liquid crystal display of the present invention, however, the power source of the capacities are made common to the power source of the wells of the semiconductor layers constituting the TFTs, whereby the connections can be simplified. In consequence, a high aperture ratio can be achieved, and a clear liquid crystal display can be obtained. Moreover, the liquid crystal display of the present invention has excellent characteristics such as a low flicker, good temperature properties, a stable operation, a clear display due to the increased aperture ratio, the enlargement of a view angle, the decrease of image deterioration by external light and a good compatibility with a panel having a large number of the pixels. In addition, when the semiconductor layers having the formed transistors comprise poly-Si or a single crystal Si, for example, a shift register, a bias regulating circuit, a gain regulating circuit or the like can be arranged at the periphery of the liquid crystal panel, so that the manufacturing cost of the panel can be decreased. The liquid crystal display of the present invention having the above-mentioned excellent characteristics can be applied to an electronic view finder for VTR incorporated into a camera, a display panel for a computer or a word processor, a head mounted display, a display panel for projection TV, a display panel for a meter such as an oscilloscope, a display panel for a handy terminal, a display panel for TV conference and TV telephone, and the like. In the accompanying drawings: Fig. 1 is an equivalent circuit illustrating one embodiment of a liquid crystal display of the present invention ; Fig. 2 is a schematic view illustrating one embodiment of an active matrix substrate which can be applied to the liquid crystal display of the present invention ; Fig. 3 is a schematic cross-section view illustrating one embodiment of a constitution of the liquid crystal display of the present invention; Fig. 4 is an equivalent circuit illustrating another embodiment of the liquid crystal display of the present invention; Fig. 5 is an equivalent circuit illustrating still another embodiment of the liquid crystal display of the present invention; Fig. 6 is a schematic plan view illustrating part of another embodiment of the active matrix substrate which can be applied to the liquid crystal display of the present invention; Fig. 7 is an enlarged plan view in which a region A in Fig. 6 is enlarged; Figs. 8A to 8J are schematic views illustrating one embodiment of a manufacturing process of a liquid crystal display embodying the present invention; Figs. 9A to 9E are schematic views illustrating initial steps of a manufacturing process of the liquid crystal display of the present invention; Fig. 10 is a schematic view illustrating still another embodiment of an active matrix substrate which can be applied to the liquid crystal display of the present invention; Fig. 11A is a schematic 1 an view illustrating part of a further embodiment of an active matrix substrate which can be applied to the liquid crystal display of the present invention; and Fig. 11B and 11C are sectional views taken along the line Y-Y' of the substrate shown in Fig. 11A. Next, the present invention will be described in particular detail with reference to typical examples. The scope of the present invention is not intended to be limited to these examples. Example 1 One embodiment of a liquid crystal display of the present invention will be described with reference to Fig. 1. Fig. 1 is an equivalent circuit for a liquid crystal display of the present invention. In Fig. 1, reference numeral 1 is a liquid crystal layer, numeral 2 is a common electrode arranged on a substrate facing an active matrix substrate on which a TFT is placed, 11, 12 and 13 are signal lines, and 21 and 22 are scanning lines. Numerals 31 and 32 are power source lines for fixing the potential of TFT wells, and they are connected in common in a periphery of a display region and further connected to a power source terminal 33. Numerals 41, 42 and 43 are sampling transistors, 51 is a horizontal signal line, 61 is a horizontal shift register, and 71 is a vertical shift register. Numerals 111, 112, 113, 121, 122 and 123 are gate electrodes of the TFTs placed in pixel divisions, and these gate electrodes are connected to the scanning lines 21, 22. Numerals 211, 212, 213, 221, 222 and 223 are regions ("wells") other than sources·drains of semiconductor layers constituting the respective TFTs, and these wells are connected to the power source lines 31, 32. Numerals 311, 312, 313, 321, 322 and 323 are sources of the respective TFTs, and they are connected to the signal lines 11, 12 and 13, respectively. Numerals 411, 412, 413, 421, 422 and 423 are drains of the respective TFTs, and they are connected to pixel electrodes 511, 512, 513, 521, 522 and 523, respectively. With regard to the TFT, n-channel (nMOS) and p-channel (pMOS) are both usable. In the active matrix type liquid crystal display, a voltage applied between the pixel electrode and the common electrode is usually about ±5 V, and between the source and the drain, a voltage of about 10 V is applied. Therefore, from the viewpoint of voltage resistance, the p- channel TFT having a small impact ionization rate is preferable. The operation of the liquid crystal display of this embodiment will be described. A video signal from a solid imaging device, a broadcast or the like is input to the horizontal signal line 51. Fig. 1 shows an example in which only one horizontal signal line is present, but in the case of a colour display or the like, a plurality of horizontal signal lines of desired colours for a red signal, a blue signal, a green signal and the like may be arranged. The video signals input to the horizontal signal line 51 are selected in turn in compliance with scanning pulses from the horizontal shift register 61 by the respective sampling transistors 41, 42, 43. On the other hand, in the case of a line to which signals should be written by the vertical shift register 71, for example, a line corresponding to the scanning line 21, pulses by which the pixel TFTs are switched on are applied to the scanning line 21. In consequence, the input video signals are written in turn to the pixel electrodes 511, 512, 513 via the signal lines 11, 12, 13. Therefore, in the case that the pixel TFTs are p-channel, a voltage is applied from the power source terminal 33 so that a well potential may be more than a maximum of the amplitude of the video signals. Furthermore, in the alternative case that the pixel TFTs are n-channel a voltage is applied from the source terminal 33 so that the well potential may be less than a minimum of the amplitude of the video signals. Immediately when one H period (one horizontal scanning period) has elapsed, the potential of the scanning line 21 is regulated by the vertical shift register 71 to be at a level at which the TFTs in the pixel divisions are switched off. The same drive is carried out for the line of the next scanning line 22. Fig. 2 shows a plan view of an active matrix substrate which can be applied to the liquid crystal display of the present invention. In Fig. 2, the same parts as in the equivalent circuit shown in Fig. 1 are represented by the same reference numerals as in Fig. 1. The signal line 11 is connected to the p + regions 311, 321 defining the sources of the pixel TFTs via contacts. Similarly, the signal line 12 is connected to the p + regions 312, 322 via contacts. The signal lines 11, 12 are each made up of a metal conductor of Al, Al-Si, Al-Si-Cu or the like. In this embodiment, the p' source and drain regions 3111, 611,312, 612 are each arranged in the vicinity of the centre of a well portion of a semiconductor layer and an n + region 1011, 1012 having a high concentration is arranged at the periphery of each well. These highly concentrated regions are held sufficiently apart from each other in consideration of voltage resistance. In the case of this embodiment, a space of 1.5 to 2.5 µm is provided between these p + and n + regions. In Fig. 2, the n wells also extend between the p + regions 311, 611, 312, 612 and the n + regions 1011, 1012, and for the relief of electric field between each n + region and p + region, p - regions (not shown) may be formed at the peripheries of the p + source and drain regions. The signals fed to the signal lines 11, 12 are transmitted through the sources to the drains of the TFTs which can be operated via the scanning line 21. In Fig. 2, highly doped p - regions 1111, 1112 are formed between the sources and channels beneath the gates, and between the drains and the channels beneath the gates in order to relieve the electric field between the sources and the channels and between the drains and the channels. Thus, even at a high voltage drive, for example, even at a level of 10 to 16 V, a sufficiently stable and reliable operation can be achieved. The scanning lines 21, 22 are usually of poly-Si, but in order to obtain a high-speed drive and satisfactory light shielding, it is desirable to use a polysilicide such as tungsten polysilicide, titanium polysilicide or molybdenum polysilicide. On the drains, drain electrodes 711, 712 are arranged, and they are connected to the p + regions 611, 612, respectively. The drain electrodes are defined by a patterned layer of the same conductor material as for the signal lines, and the semiconductor wells constituting the TFTs are covered with these wiring layers and the scanning lines, whereby the semiconductor wells constituting the TFTs are shielded from light. A signal voltage is transmitted to the pixel electrodes 511, 512 via through-holes 811, 812. This example is characterised in that, as shown in Fig. 2, the n + regions 1011, 1012 having a high concentration are arranged at the peripheries of the semiconductor wells, and these n + regions are extended along the signal lines to connect the n + regions to a desired power source, thereby to set the well potential for each of the TFTs. A sectional view cut along the line Y-Y' in Fig. 2 is shown in Fig. 3. In Fig. 3, the semiconductor well constituting a TFT is located on a glass substrate 3, and at the periphery of the semiconductor well constituting the TFT of the semiconductor layer, the n + layer portion/region 1011 is arranged. Since the TFT having a p-channel structure is used in the liquid crystal display shown in this drawing, the semiconductor layer comprises an n-type well, and since the semiconductor layer and the n + region 1011 have the same conductivity type, n, there is no problem, even if the highly concentrated n + region 1011 is not always diffused to the boundary interface between the n-type well and the glass substrate 3. In the liquid crystal display of this embodiment, there is employed a structure in which no junction region is present at the periphery of the semiconductor well, and thus no current is generated from a defective layer which is formed in the case that the junction region is placed, so that the TFT in which a leakage current is inhibited can be obtained. Next, the layer constitution of the liquid crystal display shown in Fig. 3 will be described. After the source 311, the drain 611 and the like are formed in the semiconductor layer, an interlayer insulating layer 4 and the gate electrode 21, which is continuous with the scanning line, are formed. The interlayer insulating layer can be made up of an SiN film formed by the use of a vacuum CVD method, PSG which is a phosphorous-doped glass, or a boron phosphorus-doped glass. By the way, it is very important to make uniform the liquid crystal gap with the intention of obtaining a suitable display image. A laminate which can be used to constitute the liquid crystal display panel is prepared by superposing films having tensile stress or compressive stress upon each other. The films obtained by the above-mentioned vacuum CVD method have a strong tensile stress, and so the warpage of the liquid crystal display panel can be controlled by regulating the thickness of these films to a desired value. Numerals 11 and 711 are metal conductor layers, but in order to sufficiently enhance the adhesive state of the metal conductor layers, it is desirable to form BPSG under the metal conductor layers. The metal conductor layers are each preferably prepared by superposing a film of Ti, Ta, TiW, Mo, TiN, TaN or the like upon a conductor of Al, Al-Si, Al-Si-Cu or the like. The thus prepared metal conductor layers are desirable, because the contact resistance of ITO or the like which becomes the pixel electrode is low, and ohmic properties and stability can be obtained. After this, the insulating layer of PSG or the like is deposited to flatten an etch back and the like. After the through-hole 811 is formed, the pixel electrode 511 is formed by a sputtering process. Prior to the film formation of a transparent electrode ITO (indium tin oxide) for the pixel electrode by the use of the sputtering process, the surface of the metal wire is preferably cleaned by presputtering. This cleaning operation realises a good ohmic connection between the pixel electrode 511 and the metal conductor layer 711 which is the drain electrode. After the patterning of the pixel electrode, a passivation film is formed, and a polyimide film is then applied thereonto as a liquid crystal orientation film. For the orientation, rubbing is used. The orientation film may comprise a one-layer structure or a multi-layer structure by which a view angle is enlarged. On the other hand, a faceplate substrate is produced by the following procedure. A light shield layer 10 for shading a step portion of the TFT is formed on the glass substrate 200, while the space between the pixels is regulated, and colour filter layers 9 are patterned so as to be separated by the pixels, as shown in Fig. 3, whereby a problem of colour mixing between the colour filters can be avoided. On the colour filter layer 9, a transparent electrode which is a common electrode and an orientation film 8 are formed to obtain the faceplate substrate. Between the thus obtained faceplate substrate and the active matrix substrate, a liquid crystal 7 is confined to constitute the liquid crystal display panel. In Fig. 3, polarising plates and the like are not shown, but polarising plates are arranged on the outer surfaces of both the substrates. In this embodiment, a voltage is applied to a semiconductor layer region having the same conductivity type as of the well of the semiconductor constituting the TFT to fix the well potential. Then, on the side wall of the semiconductor well constituting the TFT, an impurity region having a conductivity type opposite to that of each source and drain is formed. According to the above-mentioned constitution, the leakage current of the TFT can be reduced, so that a display having a high contrast and a high gradation can be realised. In addition, defects due to damage caused at the time of manufacture can be decreased, and a liquid crystal display panel having a stable operation can be obtained with a high yield. Example 2 An embodiment of a liquid crystal display shown in Fig. 4 will be described. Fig. 4 shows an equivalent circuit, and in Fig. 4, the same parts as in an equivalent circuit shown in Fig. 1 are represented by the same reference numerals and their detailed description will be omitted. The liquid crystal display of this embodiment is different from a liquid crystal display of Example 1 in that conductors 34, 35 for fixing a potential in a region (well) having a conductivity type different from that of each source and drain constituting a TFT are arranged along scanning lines 21, 22, and a common connection is made at the periphery of the display panel, and video signals are sampled by the use of CMOS analog switches 44, 45 and 46. When the liquid crystal display panel is driven, it is effective for the inhibition of flicker that the voltage to be applied to the liquid crystal every line is reversed. In this case, a strong electric field takes place in a line direction extending between the pixel electrodes on each line, i.e., a longitudinal direction, so that a line defect called discrimination occurs, with the result that blank areas appear irrespective of the voltage to be applied to the pixel electrode and contrast deteriorates. Therefore, for the liquid crystal display capable of displaying an image with a high contrast, it is important that a space between pixel aperture regions in the longitudinal direction is set so as to be wider than in the transverse direction. According to this embodiment, the wires 34, 35 for fixing the well potential are arranged in a horizontal direction, so that the region where the blank areas tend to appear is effectively shaded, and the reverse drive is also possible. In consequence, a pixel display having good contrast and less flicker can be realized. In the liquid crystal display of this embodiment, the sampling of the video signals is carried out by the use of the CMOS analog switches 44, 45 and 46, and the well potentials of the CMOS analog switches are also set to suitable values for the n-channel and p-channel TFTs, respectively. Reference numerals 36 and 38 are wires for fixing the well potentials, and numerals 37 and 39 are power sources for these conductors. Although not clearly shown in Fig. 4, well potentials of transistors used in peripheral circuits such as vertical and horizontal shift registers are also set to suitable values. According to the above-mentioned constitution, the video signals can be sampled and fed to the signal lines independently of threshold values of the sampling transistors and the like, and therefore a panel drive voltage can be lowered to enhance reliability owing to the enlargement of a voltage resistance margin and to increase the freedom of the design of peripheral circuits. In addition, the freedom of connections to other ICs increases, and a decrease of cost can also be achieved. Example 3 An embodiment of a liquid crystal display shown in Fig. 5 will be described. Fig. 5 shows an equivalent circuit, and in Fig. 5, the same parts as in an equivalent circuit shown in Fig. 1 are represented by the same reference numerals and their detailed description will be omitted. This embodiment is characterized in that capacitors 1211, 1212, 1213, 1221, 1222, 1223 for holding a voltage in each pixel are installed, and terminals of these capacitors are connected to the drain electrodes of TFTs and the other terminals of these capacitors are connected to electrodes 34, 35 at the well potential of the TFTs. In general, the potential of the drain terminal of the TFT changes owing to the responsibility of the liquid crystal, deflection caused by on and off of the TFT and a leakage current of the TFT. In particular, it is difficult to solve problems raised by these causes, and so an image quality often deteriorates. On the contrary, in the constitution of this embodiment, sufficient capacities of the capacitors, which are not affected by these causes, are arranged in pixel divisions, whereby a good and stable image display can be realized. The structure of the liquid crystal display of this embodiment will be described with reference to a plan view shown in Fig. 6. In the liquid crystal display panel shown in Fig. 6, signal lines 11, 12 are arranged, while shifted as much as a 1/2 pixel every line for the purpose of the improvement of resolution and colouring properties, and these signal lines are non-linear. Sources 311, 312, drains 611, 612 or gates are arranged, being protruded from scanning lines 21, 22, and the arrangement direction of each source and drain pair extends along the scanning lines. A signal voltage from the signal lines 11, 12 is transmitted to the sources 311, 312 of the TFTs, respectively, and when the voltage by which the TFT is switched on is applied to the scanning line 21, it is forwarded to the drain divisions 611, 612. Fig. 7 shows a detailed view of a periphery of a region surrounded by a dotted line a. Description will be made with reference to Fig. 7 in addition to Fig. 6. At the periphery of the source and drain portion of the TFT, a highly concentrated impurity layer 1301 having the same conductivity type as each well is formed in the upward, downward, right and left directions of the figure except a part of a lower portion in which an opening 1800 of a panel, the source, the drain, the scanning line and the signal line are arranged. As shown by numeral 1303 in Fig. 6, the potential is applied to a conductivity wiring layer at the periphery of the panel opening 1800, so that it is fixed. On the highly concentrated impurity layer 1301, a capacity forming member (plate) 1303 is formed with an insulating layer therebetween. The capacity forming member 1303 is made up of a poly-Si polysilicide, a high-melting metal or the like. This capacity forming member 1303 is used to form a capacity between the capacity forming member 1303 and the highly concentrated impurity layer 1301. The signal applied to the drain is fed to the capacity forming member 1303 by an interconnect via a through-hole. Thus, one terminal of the capacity forming member 1303 is connected to the drain of the TFT, and the other terminal of the capacity forming member 1303 is connected to the well of the TFT. Although not shown in Figs. 6 and 7, a light shield is formed covering all of the front side of this substrate except for each opening 1800. The light shield can be an insulating layer or a conductive layer. In this embodiment, a TaN film is used. In this case, the TaN film is a conductive layer, and therefore if the TaN film contacts the drain or the pixel electrode, the signal is not transmitted to the pixel electrode. Accordingly, a hole 1422 is formed around a through-hole 1522 for connecting the pixel electrode to the drain. Given the above-mentioned structure, the pixel electrode potential is more stable, and a high-quality image display can be realised. Next, a preparation method of the active matrix substrate used in the embodiment will be described with reference to Figs. 8A to 8J and 9A to 9E. Figs. 8A to 8J describe a serial of preparation steps, and Figs. 9A to 9E describe one embodiment of a process for forming a semiconductor layer on an insulating substrate. As shown in Fig. 8A, a semiconductor Si layer 3001 was formed on a glass substrate 3. The semiconductor layer may be made of a substance such as GaAs in the group III to V, or a substance in the group II to VI, instead of Si. In the case of using poly-Si, its thickness is preferably as thin as 10 to 70 nm (100 to 700 Å). To produce it, an amorphous Si is deposited from an Si 2 H 6 material gas by a vacuum CVD method at 450°C, and a heat treatment is then carried out at 600°C for 24 hours in N 2 , whereby solid phase crystallisation is done. Furthermore, the heat treatment is carried out at 1050°C to 0 2 to improve crystallinity. Alternatively, the poly-Si film may be formed from an SiH 4 material gas at a temperature of 500 to 600°C, and the ion injection of Si may be then done to bring the poly-Si film into a state close to an amorphous state. Afterwards, the film may be subjected to a heat treatment at 500 to 700°C for 50 hours. In the case of using a semiconductor layer of single crystal, its thickness is preferably as thick as 100 to 800 nm (1000 to 8000 Å), but this value is not restrictive. The single crystal was formed as follows. In the first place, an oxidized Si wafer or an Si wafer was integrally stuck on a quartz glass wafer in a clean atmosphere. Afterward, it was subjected to an anneal treatment at a temperature of 150 to 200°C and then ground by a grinder so that the thickness of an Si layer might be 100 to 200 µm. Next, the anneal temperature was raised up to 300°C, and etching was then carried out with a TMAH (tetramethylammonium hydroxide) etching solution so that the thickness of the Si layer might be about 0.5 µm. After thus thinned by the etching, the Si layer was subjected to the anneal treatment at 1000°C again to increase the mechanical strength of the substrate. In the case of such a process, the internal uniformity of the Si thin layer depends largely upon the precision of the grinder inconveniently. For the avoidance of this inconvenience, a technique shown in Figs. 9A to 9E can be utilized. This technique will be described with reference to Fig. 9A to 9E. A current of about 1 A is allowed to flow through a hydrofluoric acid-ethanol mixed solution in which an Si wafer 4001 is immersed, thereby forming a porous layer 4002 having a thickness of about 15 µm on the surface of the Si wafer 4001 (Fig. 9A). Afterwards, the porous layer is washed and then oxidized at 400°C for 20 minutes in an oxygen atmosphere, and the resultant surface oxide layer is removed in a diluted hydrofluoric acid solution. The treated wafer thus obtained is placed in a vacuum CVD device, and a heat treatment is then carried out at 1100°C for 7 minutes in an H 2 atmosphere. Afterwards,an epitaxial Si film 4003 having a thickness of 0.5 to 1 µm is allowed to grow at 900°C on the porous Si layer 4002 (Fig. 9B). Even on the porous layer, the high-quality epitaxial film can grow, if the diluted hydrofluoric acid and the H 2 anneal treatment are given. As in the above-mentioned example, this wafer is bonded to a quartz glass 4006 (Fig. 9C), and the annealing and the etching of the non-porous layer 4001 are repeated. In this case, the quartz glass 4006 to be bonded may be replaced with an Si wafer 4004 on which an oxide film 4005 is formed. For the removal of the non-porous layer 4001 of Si, grinding or polishing may be used instead of the etching. When the thickness of the non-porous layer 4001 of Si becomes about 150 µm, the remaining Si layer is removed with a hydrofluoronitic acetic acid solution. At this time, the porous Si layer 4002 is exposed, with the result that its surface blackens, whereby it is apparent that the etching progresses to such a level that the porous Si layer 4002 is reached. Instead of this process, the polishing grinder may be used to expose the porous layer. After the porous layer 4002 has been exposed, the porous layer 4002 is removed by etching in a hydrofluoric hydrogen peroxide solution (Fig. 9D), whereby the single crystal Si layer 4003 having a uniform thickness can be formed on the glass substrate 4006 (Fig. 9E). As described above, the glass substrate 4006 may be replaced with a laminate substrate obtained by forming the oxide film 4005 on the Si wafer 4004. In this case, if the Si wafer 4004 is partially etched to become light-transmittable, the thus etched Si wafer can be applied to the liquid crystal panel. Also by the above-mentioned procedure, the semiconductor thin film 3001 can be formed on the insulating substrate 3 (Fig. 8A). Next, as shown in Fig. 8B, an oxide film 1601 and an SiN film 1602 were formed except the partial regions of an opening region and a wiring region, followed by a heat treatment, whereby as shown in Fig. 8C, the region of the Si layer in which the oxide film 1601 and the SiN film 1602 were not formed was oxidized to form a field oxide film 1603. As shown in Fig. 8D, a resist mask 3002 was placed, and the field oxide film 1603 which was a part of the wiring region was removed. After the removal, ions of arsenic or phosphorus were injected through this resist mask 3002 to form an n + region at the Si thin film edge 1604. This step is not always necessary, and so if a low cost is required, the step may be omitted. Next, as shown in fig. 8E, a highly concentrated impurity layer 1605 for fixing the well potential of the TFT was formed. In order to lower the resistance of the highly concentrated layer and to shorten, to the utmost, the diffusion length of photo-carriers generated by stray light, it is desirable that the highly concentrated impurity layer 1605 reaches the insulating layer and the concentration is 10 19 cm -3 or more. Next, as a member for forming the scanning line, the gate line and the capacitor of the TFT, a poly-Si film was deposited after the formation of a gate oxide film, followed by patterning (Fig. 8F). Numeral 1606 is a member for forming the capacity (corresponding to 1303 shown in Fig. 6), 1607 is a gate of the TFT, and 1608 is a poly-Si for a scanning line region. As shown, if possible, the scanning line 1608 should not be formed on the Si semiconductor layer to lower parasitic capacity. The scanning line is arranged between the semiconductor layers, and therefore a step also decreases, so that an orientation failure reduces. Next, the above-mentioned poly-Si was used as a mask, and p- regions 1609 were formed. These are to lie between the gate and the drain and between the gate and the source so as to provide a LDD/LDS structure to relieve electric field. As shown in Fig. 8G, p + regions 1610 defining the source and the drain were formed, and in this case, these layers were offset as much as 1 to 1.5 µm to the gate 1607, and a distance from the n + region 1605 was set to 1.5 to 2.5 µm. In consequence, a voltage resistance of 30 V or more could be realised which was sufficient for liquid crystal drive. After the annealing of the p + regions 1610 of the source and drain, an SiN film 1611 having a thickness of 450 nm (4500 Å) and a BPSG 1612 having a thickness of 200 nm (2000 Å) were formed by the use of a vacuum CVD method. According to the SiN film, tensile stress can be obtained, and this film is effective to regulate the warpage of the laminate film. The control of the warpage is important to make uniform the liquid crystal gap. The BPSG has an advantage that metal wiring thereon can be formed with a good adhesion. Contact holes were formed at positions represented by reference numeral 1615 of the interlayer insulating layer comprising the SiN film 1611 and the BPSG 1612, and a wiring conductor layer 1613 was deposited. The wiring conductor layer 1613 was made from, for example, Al-Si, Al-Si-Cu or the like, and on the wiring conductor layer 1613, a layer 1614 was formed and in this case, as materials for the layer 1614, there can be used TiN, TaN, Mo, TiW, Ti, Ta and the like (Fig. 8H). This constitution can improve contact properties with a transparent electrode, is effective for the prevention of hillock to the upper layers and the improvement of light shielding, and can improve yield. As shown in Fig. 8I, after the deposition of PSG and the application of a resist, the insulating layer on the wiring conductors was flattened by etching back, and the PSG was then deposited thereon again to form an insulating layer 1616 having a total thickness of about 1000 to 1300 nm (100000 to 13000 Å). In general, in the liquid crystal display panel, shading is carried out by forming a light shield on the faceplate substrate, but in this case, an aperture ratio decreases in view of a position deviation from the active matrix substrate in which the TFTs are defined. Here, a light shield represented by reference numeral 1617 was formed on the active matrix substrate in which the TFTs are defined. In consequence, the light shield can be formed with an alignment precision of a usual semiconductor process. For example, even in a panel having a level of 3,000,000 pixels in 0.7 inch (1 inch = 2.54 cm), an aperture ratio of 30% can be obtained. As understood from the position of the light shield 1617 in Fig. 8I, the region over the pixel electrode is left open. Next, as shown in Fig. 8J, after a PSG film 1618 having a thickness of 100 to 200 nm (1000 to 2000 Å) was deposited, a through-hole through which the pixel electrode contacts the drain region is formed by etching, and a transparent electrode ITO 1619 for the pixel electrode was deposited. In order to obtain a good ohmic contact, it is important to remove the insulating layer formed on the wiring by presputtering the through-hole prior to this deposition. After the patterning of the transparent electrode 1619, a PSG film 1620 having a thickness of about 800 nm (8000 Å) was deposited as a passivation film. Here, if a step is present at the edge portion of the opening region, a blank area is formed, and therefore it is desirable to flatten this PSG film. Furthermore, if a passivation film on the pixel electrode alone is removed, baking properties are improved, thereby providing a stable display. Connection of an electrode pad to the light shielding layer and to a power source are made at a periphery of the panel. The connection is not shown in the drawings. The cross-section shown in Fig. 8J corresponds to the section of the substrate cut along the line X-X' shown in Fig. 7. Afterwards, although not shown,a polyimide film having a thickness of about 50 nm (500 Å) is formed, followed by a rubbing treatment carried out as an orientation treatment. In this case, for the improvement of the view angle of the liquid crystal, it is also effective to form a plurality of orientation films and a plurality of regions having different pretilt angles. In the liquid crystal display of this embodiment using the active matrix substrate described above, the leakage current of the TFTs could be further reduced, and the stability of the pixel potential could also be improved, whereby a good image display could be realised. In addition, as understood from Figs. 8I and 8J, the field oxide film 1603 in the opening region was thick, and a flat constitution was realised. In consequence, no blank area was present, and a high contrast and a high aperture ratio could be realised. Example 4 Description will be made with reference to Fig. 10. Fig. 10 shows a plan view of an active matrix substrate used in a liquid crystal display panel of this embodiment. In fig. 10, reference numerals 1721, 1722 are scanning lines, numerals 1711, 1712 are signal lines, 1731 is a p + source for connecting the signal line to a TFT, 1732 is a p + drain of the TFT. Reference numeral 1733 is a p - region formed to relieve an electric field of the source and gate as well as the drain and the gate, and to relieve an electric field of the source and the well as well as the drain and the well. Numeral 1734 is a highly concentrated region having the same conductivity type as the well, and in this embodiment, it is a n + region. Numeral 1735 is a contact hole for connecting the drain to a poly-Si plate (not shown) arranged over the n + layer, and 1736 is a through-hole for connecting the drain to a pixel electrode (not shown). As shown by numeral 1737, the gate of the TFT which can be used in this embodiment is divided into two on the side wall portion of the semiconductor layer, and the highly concentrated layer 1734 is placed between them. As described above, the main position of the current leakage in the TFT is the side wall portion. According to the above-mentioned structure, the gate of the side wall portion is doubled so as to have a redundancy, and even if the leakage occurs by a defect or the like at one gate, the leakage can be prevented by the other gate. Heretofore, the constitution simply comprising two gates as the redundant gate has been used, but if such a constitution is taken, a channel division of the TFT becomes a series connection, and its resistance increases twice as compared with the case of one gate. Therefore, the writing of the signal is correspondingly slow, and the size of the TFT also increases and there is a drawback that a practical aperture ratio is low. If the constitution of this embodiment is used, the leakage current is prevented more effectively than in the case that the double gate is used, and the channel resistance of the TFT is low as in the case of the single gate. Thus, the writing speed is high, and the TFT leakage is also low. In consequence, a high-quality image display can be realized, and the aperture ratio is high and a clear display is possible. As also understood from Fig. 10, the highly concentrated impurity layer n + represented by the numeral 1734 and the gate 1737 are arranged suitably apart from each other, and voltage resistance of the channel produced at the time of ON of the gate and the above-mentioned n + layer can be set at a sufficiently high level, whereby a high reliability could be obtained. Furthermore, as shown by numerals 1738, 1739 in Fig. 10, a metal layer is formed as a wiring layer or a light shield with an edge over size of the semiconductor layer. According to the present structure, the light shield metal is also arranged on the side wall portion of the semiconductor, so that there can be exerted an effect that the deterioration of contrast does not occur by light leakage. Example 5 Description will be made with reference to Figs. 11A, 11B and 11C. Fig. 11A is a plan view of an active matrix substrate used in a liquid crystal display panel of this embodiment, Fig. 11B is a sectional view cut along the line Y-Y' in Fig. 11A, and Fig. 11C is a sectional view of its variant. Reference numeral 1811 is a signal line, numeral 1831 is a p + region constituting a source 1832 is a p + region constituting a drain, 1835 is a contact to a poly-Si plage 1838 forming part of a capacitor, and 1836 is a through-hole for connecting the drain to a pixel electrode 1839. In the above-mentioned example, a well potential is drawn from a semiconductor layer on which the drain, the source and the well of a TFT are formed, and then fixed, but in this embodiment, a poly-Si layer 1837 which is different from the semiconductor on which a transistor is formed is used as a conductor for fixing the well potential, and this point is different from the above-mentioned example. In this structure, a highly concentrated impurity layer 1840 is formed in a self alignment with a direct contact 1841 by diffusion from the poly-Si layer 1837. Therefore, the semiconductor layer area of the TFT can be reduced, and a liquid crystal display which could prevent light leakage could be realized. Moreover, as understood from Fig. 11B, in this structure, as shown by numerals 1837, 1838, the two poly-Si layers are formed, and the capacity is formed between these two poly-Si layers. The poly-Si layers for setting this well potential are not necessarily constituted of the poly-Si singly, and they may comprise a mixture of a metal and Si such as a polysilicide layer. As shown in Fig. 11C, in the poly-Si 1838 of the second layer, a drain 1842 is also formed by direct contact, and contact with the pixel electrode is also directly carried out on the poly-Si 1838 without using an Al interconnect. In this case, on the poly-Si surface, a Ti layer is formed, and Ti silicide or the like is formed by a heat treatment, so that a good ohmic contact can be realised. According to the above-mentioned constitution, a space between the drain and the pixel electrode contact can be decreased, and a clear panel having a high aperture ratio can be realised. In Fig. 11C, the contact 1842 and the through-hole 1843 are formed in different positions, but if they are arranged in a longitudinal direction, a higher aperture ratio can be achieved.

Claims: An active matrix substrate, for use in a flat-panel liquid crystal display, comprising: a substrate (3) having thereon a plurality of pixel electrodes (511-513; 521-523), each connected to the drain (411-413, 421-423) of a respective field-effect transistor defined therein, and arranged by row and by column in a matrix array; respective scanning lines (21,22), arranged along each column, connected to the gates (111-113, 121-123) of the transistors of the respective columns; and respective signal lines (11,12,13), arranged along each row, connected to the sources (311 & 321, 312 & 322, 313 & 323) of the transistors of the respective rows; characterised by: a layer (1011) of semiconductor having a high density of dopant impurities (n + ), which is arranged around the periphery of, and in contact with, semiconductor layer portions (211-213, 221-223; 911) forming the field effect transistors; which layer (1011) of semiconductor is of conductivity type (n) opposite to that (p) of the source and drain of each transistor, and is provided with means of connection (31-33) to a source of fixed voltage. A substrate according to claim 1 wherein said layer of semiconductor having said high density of dopant impurities includes line portions (31-33; 34,35) adjacent, and extending in directions parallel to, either said signal lines or said scanning lines, to the periphery of said matrix array, which line portions are co-extensive with portions of said layer of semiconductor at the periphery of each field-effect transistor in said rows or columns, and provide said means of connection to the source of fixed voltage. A substrate according to either preceding claim having a respective capacitor (1211-1213, 1221-1223) connected between the drain and the semiconductor layer portion of each said field-effect transistor. A substrate according to claim 3 wherein each said respective capacitor has one plate (1303) thereof connected to the respective drain and the other plate thereof constituted by a respective part of said layer (1301) of semiconductor having said high density of dopant impurities. A substrate according to claim 3 wherein each said respective capacitor has one plate (1838) thereof connected to the respective drain (1832) and the other plate (1837) thereof constituted by a respective plate forming layer (1837), which is not part of said semiconductor layer portion forming the respective field-effect transistor, which respective plate forming layer (1837) is in contact with said layer (1840) of semiconductor having said high density of dopant impurities. A substrate according to any preceding claim wherein the source and drain of each said field-effect transistor are inset from the edge of the semiconductor layer portion forming that field-effect transistor. A substrate according to claim 1 having a patterned layer of semiconductor (1837), other than said layer of semiconductor having a high density of dopant impurities, provided as said means of connection to the source of fixed voltage. A substrate according to any preceding claim wherein the respective gate electrode (1721) of each respective said field-effect transistor has a bifurcated portion (1737) at the edge of the respective semiconductor layer portion forming the respective field-effect transistor. A substrate according to any preceding claim wherein said semiconductor layer portions forming said field-effect transistors are portions of a polycrystalline or monocrystalline silicon layer. A substrate according to claim 9 wherein said semiconductor layer portions are portions of a polycrystalline silicon layer of thickness 10 to 70 nm. A substrate according to claim 9 wherein said semiconductor layer portions are portions of a monocrystalline silicon layer of thickness 100 to 800 nm. A substrate according to any preceding claim wherein said field-effect transistors are thin film transistors. A substrate according to any preceding claim wherein said respective sources and drains each have a lightly doped portion adjacent to the respective channel therebetween. A substrate according to claim 13 wherein said respective sources and drains have a lightly doped portion adjacent to the respective semiconductor layer portions extending between said sources and drains and said layer of semiconductor having a high density of dopant impurities which is arranged at the periphery of said respective semiconductor layer portions. A liquid crystal display panel comprising: an active matrix substrate (3) according to any of claims 1 to 14; a faceplate substrate (200) having a common electrode (9) arranged opposite to said active matrix substrate with said common electrode (9) and the pixel electrodes (511) of said active matrix substrate arranged facing inwardly; and a layer (7) of liquid crystal material sealed between said active matrix substrate and said faceplate substrate.

Inventors: MIYAWAKI MAMORU, MIYAWAKI, MAMORU, MIYAWAKI, MAMORU,C/O CANON K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1994
Main classification: G02F1
IPC codes: G02F1, H01L29, H01L27, G09G3